Digital imaging devices, such as digital cameras, allow users to take photographs and store them in digital form. In general, digital imaging devices capture images by exposing an optical sensor, such as a Charged Coupled Device (CCD), to a scene for a particular exposure time. As digital imaging technology advances, CCDs are able to capture images with greater resolution. The resolution of a digital camera refers to the number of pixels included in a captured image. For example, a three-megapixel digital camera takes an image that is divided into three million pixels. As the pixel size decreases, it is increasingly important to ensure that each pixel is exposed to a sufficient amount of light to capture the image. For instance, the exposure time may be increased to ensure that each pixel captures enough light.
In general, typical digital imaging devices do not have enough sensitivity to capture images in many low light situations. For example, a user may wish to take photos in a museum or at a performance. In many of these cases, the user is not permitted to use a flash when taking the picture. Typically, the digital imaging device will set a very long exposure time (e.g., several seconds) to ensure that enough light is captured. However, the digital imaging device typically captures motion blurred images because the user cannot hold the imaging device steady enough during the course of the exposure. Furthermore, the subject of the photo may also move during the exposure, further blurring the captured image.
Some digital imaging devices also provide users with telephoto options to zoom in on a scene, enabling the capture of a closer version of the scene. As the zoom factor for capturing an image increases, the exposure time is typically proportionally shortened. However, as the pixel sizes decreases in higher resolution imaging devices, the exposure time may not be proportionally shortened, or shortened at all, to ensure that enough light is captured.
In general, there is a trade-off between shorter exposure images and longer exposure images. A short exposure image is typically sharp, as there is little motion blur. However, short exposure images are also typically noisy, as the signal-to-noise-ratio is low for underexposed pixels. Signal-to-noise-ratio decreases as the pixels in the image sensors receive less photons and short exposure images may not receive sufficient photons to ensure high signal-to-noise-ratio. In contrast, a long exposure image is well exposed and less noisy. However, as described above, long exposure images are subject to motion blur, resulting in blurred images.
In the prior art, it has been proposed to capture two or more images, including a short exposure image and a long exposure image, and to select pixels of the two images based on motion detection. Once the two images are captured, it is determined whether there was any motion for each pixel. If there was any motion for a pixel, the short exposure pixel is selected, and if there was not motion for a pixel, the long exposure pixel is selected. The final image will include both under-exposed and noisy pixels from the short exposure image, and blurred pixels from the long exposure image. Thus, if there was movement for all pixels, the shorter exposure image, which is noisier than the long exposure image, is selected. This method is a passive approach, simply selecting pixels from the two images, and does not deblur either image.